Bipolar mapping of intracardiac potentials using recessed electrodes

ABSTRACT

A method for measuring near-field electrical activity at a location in a heart comprising introducing into the heart a catheter. The catheter comprises an elongated tubular body having a distal region and a circumferential recess along the length of the distal region, a first electrode mounted on the distal region in close proximity to the circumferential recess, and a second electrode mounted within the circumferential recess. The distal region is positioned at the location in the heart so that the first electrode is in direct contact with heart tissue and the second electrode is not in direct contact with heart tissue but is in contact with blood. A first signal is obtained with the first electrode, and a second signal is obtained with the second electrode. The first signal and the second signal are compared to obtain the near-field electrical activity at the location in the heart.

FIELD OF THE INVENTION

[0001] The present invention is directed to a method for measuringelectrical activity in the heart and a catheter useful for performingthe method.

[0002] The present invention is related to other commonly owned U.S.patent applications: U.S. Serial No. ______, entitled Catheter with TipElectrode Having a Recessed Ring Electrode Mounted Thereon; U.S. Ser.No. ______, entitled Mapping and Ablation Catheter; U.S. Ser. No.______, entitled Multi-Electrode Catheter, System and Method; and U.S.Ser. No. ______, entitled System and Method for DetectingElectrode-Tissue Contact; all commonly owned by the assignee of thepresent invention and the disclosures of each are incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION

[0003] Electrode catheters have been in common use in medical practicefor many years. They are used to stimulate and map electrical activityin the heart and to ablate sites of aberrant electrical activity.

[0004] In use, the electrode catheter is inserted into a major vein orartery, e.g., femoral artery, and then guided into the chamber of theheart which is of concern. Within the heart, the ability to control theexact position and orientation of the catheter tip is critical andlargely determines how useful the catheter is.

[0005] In healthy humans the heartbeat is controlled by the sinoatrialnode (“S-A node”) located in the wall of the right atrium. The S-A nodegenerates electrical signal potentials that are transmitted throughpathways of conductive heart tissue in the atrium to theatrioventricular node (“A-V node”) which in turn transmits theelectrical signal potentials throughout the ventricle by means of theHis and Purkinje conductive tissues. Improper growth of or damage to theconductive tissue in the heart can interfere with the passage of regularelectrical signals from the S-A and A-V nodes. Electrical signalirregularities resulting from such interference can disturb the normalrhythm of the heart and cause an abnormal rhythmic condition referred toas cardiac arrhythmia.

[0006] Electrophysiological ablation is a procedure often successful interminating cardiac arrhythmia. This procedure involves applyingsufficient energy to the interfering tissue to ablate that tissue thusremoving the irregular signal pathway. However, before the ablationprocedure can be carried out, the interfering tissue must first belocated.

[0007] One location technique involves an electrophysiological mappingprocedure whereby the electrical signals emanating from the conductiveendocardial tissues are systematically monitored and a map is created ofthose signals. By analyzing that map, the interfering electrical pathwaycan be identified. A conventional method for mapping the electricalsignals from conductive heart tissue is to percutaneously introduce anelectrophysiology catheter (electrode catheter) having mappingelectrodes mounted on its distal extremity. The catheter is maneuveredto place these electrodes in contact with or in close proximity to theendocardium. By monitoring the electrical signals at the endocardium,aberrant conductive tissue sites responsible for the arrhythmia can bepinpointed.

[0008] Once the origination point for the arrhythmia has been located inthe tissue, the physician may use an ablation procedure to destroy thetissue causing the arrhythmia in an attempt to remove the electricalsignal irregularities and restore normal heart beat or at least animproved heart beat. Successful ablation of the conductive tissue at thearrhythmia initiation site usually terminates the arrhythmia or at leastmoderates the heart rhythm to acceptable levels.

[0009] Conventional unipolar electrode catheters utilize a primary tipor ring electrode that cooperates with a reference electrode outside thepatient's body. Such catheters are known to map inaccurate electricalreadings due to the reference electrode being located outside thepatient's body.

[0010] Previous attempts have been made to design a bipolar electrodecatheter having two electrodes within the patient's body. However, suchcatheters also have limited accuracy. Specifically, both electrodes pickup near field electrical signals emanating from the conductiveendocardial tissues due to their contact with the heart tissue, andfar-field electrical signals which propagate from other regions of theheart due to their contact with the blood. The far-field signalsinterfere with the near-field signals, making accurate measurement ofthe near-field signals difficult. Accordingly, a need exists for abipolar electrode catheter that more accurately measures near-fieldsignals.

[0011] U.S. Pat. No. 5,749,914 to Janssen discloses a catheter forremoving obstructions from a tubular passageway in a patient. In oneembodiment, Janssen describes a catheter having a distal end with arecessed annular ridge that defines a groove in which a plurality ofelectrodes are seated. The electrodes are sized so that they arerecessed within the annular ridge. A return electrode is located on thecatheter proximal to the recessed electrodes. The electrodes areconnected to a radio-frequency energy source that generates and suppliescurrent to the electrodes to ablate constructive material. Janssennowhere teaches or suggests, however, using this catheter to mapelectrical activity in the heart.

[0012] U.S. Pat. No. 4,966,597 to Cosman discloses a cardiac ablationelectrode catheter with a thermosensing detector at a position in thedistal end of the catheter. In one embodiment, the ablation electrodehas an insulative exterior with openings that provide exposed electrodesurfaces. Each of the electrode surfaces can be independently connectedto different contacts, which are then connected to a voltage source, orthe electrode surfaces can all be connected together. Atemperature-measuring conductor is attached to one or more of theelectrode surfaces. The object of the invention described in Cosman isto provide a cardiac catheter for tissue ablation with ultra-fastfaithful recording of temperature in the affected tissue. Cosman nowherediscloses, however, obtaining electrical signals with differentelectrodes and comparing the signals to obtain near-field electricalactivity information.

SUMMARY OF THE INVENTION

[0013] The present invention is directed to a catheter having twoelectrodes for bipolar mapping and a method for using the catheter. Inone embodiment, the invention is directed to a method for measuringnear-field electrical activity at a location in a heart. The methodcomprises introducing into the heart a catheter comprising an elongatedtubular body having a distal region and a circumferential recess alongthe length of the distal region. A first electrode is mounted on thedistal region in close proximity to the circumferential recess. A secondelectrode is mounted within the circumferential recess. The methodfurther comprises positioning the distal region at the location in theheart so that the first electrode is in direct contact with heart tissueand the second electrode is not in direct contact with heart tissue butis in contact with blood. A first signal is obtained with the firstelectrode, and a second signal is obtained with the second electrode.The first signal and the second signal are compared to obtain thenear-field electrical activity at the location in the heart.

[0014] In another embodiment, the invention is directed to a method formeasuring near-field electrical activity at a location in a heartcomprising introducing into the heart a catheter comprising an elongatedbody having an outer diameter and a distal region, a first electrodemounted on the distal region, and a second electrode mounted on thedistal region in close proximity to and electrically isolated from thefirst electrode, the second electrode having an outer diameter less thanthe outer diameter of the portion of the distal region on which it ismounted. The distal region is positioned at the location in the heart sothat the first electrode is in direct contact with heart tissue and thesecond electrode is not in direct contact with heart tissue but is incontact with blood. A first signal is obtained with the first electrode,and a second signal is obtained with the second electrode. The firstsignal and the second signal are compared to obtain the near-fieldelectrical activity at the location in the heart.

[0015] In still another embodiment, the invention is directed to amethod for measuring near-field electrical activity at a location in aheart comprising introducing into the heart a catheter comprising anelongated body having a distal region, a first electrode mounted on thedistal region, and a second electrode mounted on the distal region inclose proximity to and electrically isolated from the first electrode.The second electrode is covered by a blood-permeable membrane thatprohibits direct contact between the second electrode and surroundingheart tissue. The distal region is positioned at the location in theheart so that the first electrode is in direct contact with heart tissueand the second electrode is not in direct contact with heart tissue butis in contact with blood. A first signal is obtained with the firstelectrode, and a second signal is obtained with the second electrode.The first signal and the second signal are compared to obtain thenear-field electrical activity at the location in the heart.

[0016] In yet another embodiment, the invention is directed to acatheter comprising an elongated body having a distal region. A firstelectrode is mounted on the distal region. A second electrode is mountedon the distal region in close proximity to and electrically isolatedfrom the first electrode. The second electrode is covered by ablood-permeable membrane that, in use, prohibits direct contact betweenthe second electrode and surrounding heart tissue.

DESCRIPTION OF THE DRAWINGS

[0017] These and other features and advantages of the present inventionwill be better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings wherein:

[0018]FIG. 1 is a side view of an embodiment of the catheter of theinvention.

[0019]FIG. 2 is a side cross-sectional view of a catheter body accordingto the invention, including the junction between the catheter body andtip section.

[0020]FIG. 3 is a side cross-sectional view of a catheter tip sectionshowing a tip electrode and a recessed ring electrode.

[0021]FIG. 4 is a side cross-sectional view of an alternative tipsection according to the invention having a ring electrode covered by ablood-permeable material.

[0022]FIG. 5A is a side cross-sectional view of another alternative tipsection according to the invention having a first ring electrode and asecond ring electrode that is recessed.

[0023]FIG. 5B is a side cross-sectional view of another alternative tipsection according to the invention having a first ring electrode and asecond ring electrode that is covered by a blood-permeable membrane.

[0024]FIG. 6 is a side cross-sectional view of another alternative tipsection according to the invention, the tip section including anelectromagnetic location sensor.

[0025]FIG. 7 is an end cross-sectional view of the tip section depictedin FIG. 6.

DETAILED DESCRIPTION

[0026] In a particularly preferred embodiment of the invention, there isprovided a steerable catheter having two electrodes for making bipolarmeasurements. As shown in FIGS. 1 to 3, catheter 10 comprises anelongated catheter body 12 having proximal and distal ends, a tipsection 14 at the distal end of the catheter body 12, and a controlhandle 16 at the proximal end of the catheter body 12.

[0027] With reference to FIG. 2, the catheter body 12 comprises anelongated tubular construction having a single, axial or central lumen18. The catheter body 12 is flexible, i.e., bendable, but substantiallynon-compressible along its length. The catheter body 12 can be of anysuitable construction and made of any suitable material. A presentlypreferred construction comprises an outer wall 22 made of apolyurethane, or PEBAX. The outer wall 22 comprises an imbedded braidedmesh of high-strength steel, stainless steel or the like to increasetorsional stiffness of the catheter body 12 so that, when the controlhandle 16 is rotated, the tip section 14 of the catheter 10 will rotatein a corresponding manner. The outer diameter of the catheter body 12 isnot critical, but is preferably no more than about 8 french (1 mm=3french), more preferably about 7 french, still more preferably about 5french. Likewise the thickness of the outer wall 22 is not critical, butis thin enough so that the central lumen 18 can accommodate an infusiontube, a puller wire, lead wires, and any other wires, cables or tubes.The inner surface of the outer wall 22 is lined with a stiffening tube20, which can be made of any suitable material, such as polyimide ornylon. The stiffening tube 20, along with the braided outer wall 22,provides improved torsional stability while at the same time minimizingthe wall thickness of the catheter, thus maximizing the diameter of thecentral lumen 18. The outer diameter of the stiffening tube 20 is aboutthe same as or slightly smaller than the inner diameter of the outerwall 22. Polyimide tubing is presently preferred for the stiffening tube20 because it may be very thin walled while still providing very goodstiffness. This maximizes the diameter of the central lumen 18 withoutsacrificing strength and stiffness. A particularly preferred catheterhas an outer wall 22 with an outer diameter of from about 0.090 inch toabout 0.098 inch and an inner diameter of from about 0.061 inch to about0.065 inch and a polyimide stiffening tube 20 having an outer diameterof from about 0.060 inch to about 0.064 inch and an inner diameter offrom about 0.051 inch to about 0.056 inch. If desired, the stiffeningtube 20 can be eliminated. As would be recognized by one skilled in theart, the catheter body construction can be modified as desired.

[0028] As shown in FIG. 3, the tip section 14 comprises a short sectionof tubing 19 having two lumens 30 and 32. The tubing 19 is made of asuitable non-toxic material that is preferably more flexible than thecatheter body 12. A presently preferred material for the tubing 19 isbraided polyurethane, i.e., polyurethane with an embedded mesh ofbraided high-strength steel, stainless steel or the like. The outerdiameter of the tip section 14, like that of the catheter body 12, ispreferably no greater than about 8 french, more preferably 7 french,still more preferably about 5 french. The size of the lumens is notcritical and can vary depending on the specific application.

[0029] A preferred means for attaching the catheter body 12 to the tipsection 14 is illustrated in FIG. 2. The proximal end of the tip section14 comprises an outer circumferential notch 24 that receives the innersurface of the outer wall 22 of the catheter body 12. The tip section 14and catheter body 12 are attached by adhesive (e.g., polyurethane glue)or the like. Before the tip section 14 and catheter body 12 areattached, however, the stiffening tube 20 is inserted into the catheterbody 12. The distal end of the stiffening tube 20 is fixedly attachednear the distal end of the catheter body 12 by forming a glue joint (notshown) with polyurethane glue or the like. Preferably a small distance,e.g., about 3 mm, is provided between the distal end of the catheterbody 12 and the distal end of the stiffening tube 20 to permit room forthe catheter body 12 to receive the notch 24 of the tip section 14. Aforce is applied to the proximal end of the stiffening tube 20, and,while the stiffening tube 20 is under compression, a first glue joint(not shown) is made between the stiffening tube 20 and the outer wall 22by a fast drying glue, e.g. Super Glue®. Thereafter a second glue joint(not shown) is formed between the proximal ends of the stiffening tube20 and outer wall 22 using a slower drying but stronger glue, e.g.,polyurethane.

[0030] At the distal end of the tip section 14 is a tip electrode 36.Preferably the tip electrode 36 has a diameter about the same as theouter diameter of the tubing 19. The tip electrode 36 can be made fromany suitable material, such as platinum, gold, iridium or stainlesssteel, and is preferably machined from platinum-iridium bar (90%platinum/10% iridium).

[0031] A preferred tip electrode has a length ranging from about 2.5 mmto about 8 mm, preferably about 3.5 mm. Preferably the tip electrode 36is attached to the tubing 19 by polyurethane glue or the like. The wiresthat extend into the tip electrode 36, described in more detail below,help to keep the tip electrode in place on the tubing 19 of the tipsection 14.

[0032] In the embodiment shown in FIG. 3, there is a ring electrode 39mounted within a circumferential recess 26 in the tubing 19 of the tipsection 14. The recess 26 is located near the distal end of the tipsection 14 and in close proximity to the tip electrode 36. As usedherein, “in close proximity” means a distance suitable for conductingbipolar mapping. Preferably the recess 26 is spaced apart from the tipelectrode 36 a distance no greater than about 4 mm, more preferably fromabout 0.1 mm to about 2 mm, still more preferably from about 0.5 mm toabout 1.0 mm. The width and depth of the recess 26 are designed suchthat, when the tip section 14 is positioned on its side against theadjacent heart tissue, the tissue des not come into contact with thering electrode 39. Preferably, the width of the recess 26 ranges fromabout 0.5 mm to about 4 mm, more preferably from about 1 mm to about 3mm, with the depth of the recess 26 preferably ranging from about 0.25mm to about 1.5 mm, more preferably from about 0.5 mm to about 1 mm.

[0033] In a preferred embodiment, the ring electrode 39 comprises aresilient ribbon-shaped conductive material that is wrapped within therecess 26 and fixed in place by glue or the like. The ring electrode 39can be made of any suitable conductive material, such as those discussedabove for the tip electrode. The width of and thickness of the ringelectrode 39 are suitable for fitting within the recess 26 so that theouter surface of the ring electrode 39 is recessed within the recess 26.In other words, the ring electrode 39 has an outer diameter less thanthe outer diameter of the tubing 19 of the tip section 14. Preferably,the outer diameter of the ring electrode 39 is at least about 10%, morepreferably from about 20% to about 50%, less than the outer diameter ofthe portion of the tip section 14 on which it is mounted. The ringelectrode 39 has a width preferably ranging from about 0.5 mm to about 4mm, more preferably from about 1 mm to about 3 mm. In an alternativeembodiment, the ring electrode 39 is in the form of a snap ring, wherethe width and thickness of the ring 39 are suitable for fitting withinthe recess 26, as described above.

[0034] The tip electrode 36 and ring electrode 39 are each connected toa separate lead wire 44. The lead wires 44 extend through the firstlumen 30 of tip section 14, the central lumen 18 of the catheter body12, and the control handle 16, and terminate at their proximal end in aninput jack (not shown) that may be plugged into an appropriate signalprocessing unit (not shown). The portion of the lead wires 44 extendingthrough the central lumen 18 of the catheter body 12, control handle 16and proximal end of the tip section 14 may be enclosed within aprotective sheath 49, which can be made of any suitable material,preferably polyimide. The protective sheath 49 is preferably anchored atits distal end to the proximal end of the tip section 14 by gluing it inthe first lumen 30 with polyurethane glue or the like.

[0035] The lead wires 44 are attached to the tip electrode 36 and ringelectrode 39 by any conventional technique. Connection of a lead wire 44to the tip electrode 36 is accomplished, for example, by soldering thelead wire 44 into a first blind hole 31 of the tip electrode, as shownin FIG. 3.

[0036] Connection of a lead wire 44 to a ring electrode 39 is preferablyaccomplished by first making a small hole through the tubing 19. Such ahole can be created, for example, by inserting a needle through thetubing 19 and heating the needle sufficiently to form a permanent hole.A lead wire 44 is then drawn through the hole by using a microhook orthe like. The ends of the lead wire 44 are then stripped of any coatingand soldered or welded to the underside of the ring electrode 39, whichis then slid into position over the hole and fixed in place withpolyurethane glue or the like.

[0037] A puller wire 50 extends through the catheter body 12, isanchored at its proximal end to the control handle 16, and is anchoredat its distal end to the tip section 14. The puller wire 50 is made ofany suitable metal, such as stainless steel or Nitinol, and ispreferably coated with Teflon® or the like. The coating impartslubricity to the puller wire 50. The puller wire 50 preferably has adiameter ranging from about 0.006 to about 0.010 inches.

[0038] A compression coil 52 is situated within the catheter body 12 insurrounding relation to the puller wire 50. The compression coil 52extends from the proximal end of the catheter body 12 to the proximalend of the tip section 14. The compression coil 52 is made of anysuitable metal, preferably stainless steel. The compression coil 52 istightly wound on itself to provide flexibility, i.e., bending, but toresist compression. The inner diameter of the compression coil 52 ispreferably slightly larger than the diameter of the puller wire 50. TheTeflon® coating on the puller wire 50 allows it to slide freely withinthe compression coil 52. If desired, particularly if the lead wires 44are not enclosed by a protective sheath 49, the outer surface of thecompression coil 52 can be covered by a flexible, non-conductive sheath46, e.g., made of polyimide tubing, to prevent contact between thecompression coil 52 and any other wires within the catheter body 12.

[0039] The compression coil 52 is anchored at its proximal end to theproximal end of the stiffening tube 20 in the catheter body 12 by gluejoint 51 and at its distal end to the tip section 14 by glue joint 53.Both glue joints 51 and 53 preferably comprise polyurethane glue or thelike. The glue may be applied by means of a syringe or the like througha hole made between the outer surface of the catheter body 12 and thecentral lumen 18. Such a hole may be formed, for example, by a needle orthe like that punctures the outer wall 22 of the catheter body 12 andthe stiffening tube 20 which is heated sufficiently to form a permanenthole. The glue is then introduced through the hole to the outer surfaceof the compression coil 52 and wicks around the outer circumference toform a glue joint about the entire circumference of the compression coil52.

[0040] The puller wire 50 extends into the second lumen 32 of the tipsection 14. The puller wire 50 is anchored at its distal end to the tipelectrode 36 within a second blind hole 33 by weld or the like. Apreferred method for anchoring the puller wire 50 within the tipelectrode 36 is by crimping metal tubing 54 to the distal end of thepuller wire 50 and soldering the metal tubing 54 inside the second blindhole 33. Anchoring the puller wire 50 within the tip electrode 36provides additional support for the tip electrode on the flexibleplastic tubing 19, reducing the likelihood that the tip electrode willseparate from the tubing. Alternatively, the puller wire 50 can beattached to the side of the tip section 14. Such a design is describedin U.S. patent application Ser. No. 08/924,611 (filed Sep. 5, 1997), thedisclosure of which is incorporated herein by reference. Within thesecond lumen 32 of the tip section 14, the puller wire 50 extendsthrough a plastic, preferably Teflon®, sheath 56, which prevents thepuller wire 50 from cutting into the wall of the tubing 19 when the tipsection is deflected.

[0041] Longitudinal movement of the puller wire 50 relative to thecatheter body 12, which results in deflection of the tip section 14, isaccomplished by suitable manipulation of the control handle 16. Asuitable control handle design for use with the present invention isdescribed in allowed U.S. patent application Ser. No. 08/982,113, filedDec. 1, 1997, the disclosure of which is incorporated herein byreference.

[0042] In operation, the present invention is ideal for mapping theheart and ablating accessory signal pathways causing arrhythmias. Toperform this function, the distal end of the catheter 10 is insertedinto a vein or artery and advanced into the heart. To assist inpositioning the tip section 14 of the catheter 10 at a desired positionwithin the heart, the puller wire 50 and control handle 16 are used todeflect the tip section 14. Once the tip section 14 has been positionedat or near the desired location of the heart tissue, the electricalactivity of the heart may be identified, evaluated or mapped, andelectrophysiological sources of arrhythmia may be identified and/ortreated.

[0043] Electrical activity within the heart is detected using the tipelectrode 36 and ring electrodes 39 of the catheter 10. The catheter 10of the present invention is designed such that the tip electrode 36 isin direct contact with the heart tissue. Thus, the tip electrode 36senses both the local activation energy (near-field signals) at thepoint of contact with the heart tissue and far field activation energy(far-field signals) received by the electrode through the blood.

[0044] As described above, the ring electrode 39 is recessed relative tothe tip section 14 to be protected from direct contact with the hearttissue, but permitting contact with surrounding blood. The closeproximity of the ring electrode 39 to the tip electrode 36 enables thering electrode 36 to receive approximately the same far-field signals asthe tip electrode 36. However, the ring electrode 39 does not pick upthe local activation potential (near-field signals). The signalsreceived by the tip electrode 36 and the ring electrode 39 are sent to asuitable signal processing unit.

[0045] Within the signal processing unit, the signal detected by thering electrode 39, which is only far-field signals, is subtracted fromthe signal detected by the tip electrode 36, which includes bothnear-field and far-field signals. Thus, the near-field signals can bemore accurately determined. This improved method of detecting electricalactivity allows the physician or operator to determine the location ofthe arrhythmiogenic focus more accurately for ablating and otherpurposes.

[0046] Alternate bipolar electrode designs can also be provided havingone electrode in contact with blood but not the heart tissue. Forexample, as shown in FIG. 4, the ring electrode 39 is covered by amembrane 60 that is permeable to the blood, but that prevents directphysical contact between the ring electrode and the heart tissue. Inthis embodiment the ring electrode 39 is mounted on the tubing 19proximal to and in close proximity to the tip electrode 39. The ringelectrode 39 is slid over the tubing 19 and fixed in place by glue orthe like. The membrane 60 is wrapped around the ring electrode 39 andglued in place onto the tip section 14 by polyurethane or the like. Themembrane 60 is preferably in the form of a perforated film or a woven ornonwoven fabric. The membrane 60 preferably comprises a biocompatiblepolymer. Examples of suitable biocompatible polymers for use inconnection with the invention include polyolefins such as polypropylene,polyurethane, polyetheramide, polyetherimide, polyimide, fluoropolymerssuch as polytetrafluoroethylene, silicones and the like, andcombinations thereof. The blood-permeable membrane 60 thus allows theblood to permeate the membrane 60 and contact the ring electrode 39,while protecting the ring electrode 39 from direct contact with theheart tissue.

[0047] In alternative embodiments, as shown in FIGS. 5A and 5B, ringelectrode pairs may be provided instead of the tip electrode/ringelectrode combinations described above. In these embodiments, the ringelectrode pair 61 includes first and second ring electrodes 64 and 66mounted in close proximity to each other. In one alternative embodiment,the first ring electrode 64 is mounted on the outer surface of thetubing 19 to make direct contact with adjacent heart tissue. The secondring electrode 66 is displaced within a recess 65 proximal to the firstelectrode 64 such that the second electrode 66 is recessed from theouter surface of the tubing 19 to prevent direct contact with adjacentheart tissue, in a manner as described above.

[0048] In another alternative embodiment, the first ring electrode 64 ismounted on the outer surface of the tubing 19, to make direct contactwith adjacent heart tissue. The second ring electrode 66 is mounted onthe tubing 19 proximal to the first electrode 64. A blood-permeablemembrane 60 is wrapped around the second electrode 66, in a manner asdescribed above, to protect the second electrode 66 from direct contactwith adjacent heart tissue.

[0049] As would be recognized by one skilled in the art, the relativelocations of the ring electrodes can vary. For example, in theembodiment of FIG. 5A, the second electrode 66, which is recessed, canbe distal to the first electrode 64. Also, additional ring electrodescan be provided for any of the above-described embodiments.

[0050] In an alternative embodiment, the catheter further includes alocation sensor, preferably an electromagnetic location sensor. As shownin FIGS. 6 and 7, the tip section 14 includes a third lumen 34. Theelectromagnetic sensor 72 is mounted in part in the distal end of thetubing 19 and in part in a blind hole in the tip electrode 36. Suitableelectromagnetic sensors for use in connection with the present inventionare described in U.S. patent application Ser. No. 09/160,063 (entitled“Miniaturized Position Sensor”) and U.S. Pat. Nos. 5,558,091, 5,443,489,5,480,422, 5,546,951, 5,568,809, and 5,391,199, the disclosures of whichare incorporated herein by reference. The electromagnetic sensor 72 isconnected to a electromagnetic sensor cable 74, which extends throughthe third lumen 34 of the tip section 14, through the central lumen 18of the catheter body 12, and into the control handle 16. Theelectromagnetic sensor cable 74 then extends out the proximal end of thecontrol handle 16 within an umbilical cord (not shown) to a sensorcontrol module (not shown) that houses a circuit board (not shown).Alternatively, the circuit board can be housed within the control handle16, for example, as described in U.S. patent application Ser. No.08/924,616, entitled “Steerable Direct Myocardial RevascularizationCatheter”, the disclosure of which is incorporated herein by reference.The electromagnetic sensor cable 74 comprises multiple wires encasedwithin a plastic covered sheath. In the sensor control module, the wiresof the electromagnetic sensor cable are connected to the circuit board.The circuit board amplifies the signal received from the electromagneticsensor and transmits it to a computer in a form understandable by thecomputer by means of the sensor connector at the proximal end of thesensor control module. Also, because the catheter is designed for singleuse only, the circuit board preferably contains an EPROM chip whichshuts down the circuit board approximately 24 hours after the catheterhas been used. This prevents the catheter, or at least theelectromagnetic sensor, from being used twice. If desired, the sensor 72can be contained within a rigid plastic housing, e.g., made ofpolyetheretherketone (PEEK), that is mounted between the tip electrode36 and the flexible tubing 19. Such a design is described in U.S. Pat.No. 5,938,603, the disclosure of which is incorporated herein byreference.

[0051] To use the electromagnetic sensor 72, the patient is placed in amagnetic field generated, for example, by situating under the patient apad containing coils for generating a magnetic field. A referenceelectromagnetic sensor is fixed relative to the patient, e.g., taped tothe patient's back, and the catheter containing a the electromagneticlocation sensor is advanced into the patient's heart. Each sensorpreferably comprises three small coils which in the magnetic fieldgenerate weak electrical signals indicative of their position in themagnetic field. Signals generated by both the fixed reference sensor andthe second sensor in the heart are amplified and transmitted to acomputer which analyzes the signals and then displays the signals on amonitor. By this method, the precise location of the sensor in thecatheter relative to the reference sensor can be ascertained andvisually displayed. The sensor can also detect displacement of thatcatheter that is caused by contraction of the heart muscle. A preferredmapping system includes a catheter comprising multiple electrodes and anelectromagnetic sensor, such as the NOGA-STAR catheter marketed byBiosense Webster, Inc., and means for monitoring and displaying thesignals received from the electrodes and electromagnetic sensor, such asthe Biosense-NOGA system, also marketed by Biosense Webster, Inc.

[0052] Using this technology, the physician can visually map a heartchamber. This mapping is done by advancing the catheter tip into a heartchamber until contact is made with the heart wall. This position isrecorded and saved. The catheter tip is then moved to another positionin contact with the heart wall and again the position is recorded andsaved. By combining the electromagnetic sensor and electrodes, aphysician can simultaneously map the contours or shape of the heartchamber and the electrical activity of the heart.

[0053] If desired, the catheter can be multidirectional, i.e., havingtwo or more puller wires to enhance the ability to manipulate the tipsection in more than one direction or to form two or more differentcurves. A description of such a design is described in U.S. PatentApplication Serial Nos. 08/924,611 (filed Sep. 5, 1997), 09/130,359(filed Aug. 7, 1998), 09/143,426 (filed Aug. 28, 1998), 09/205,631(filed Dec. 3, 1998), and 09/274,050 (filed Mar. 22, 1999), thedisclosures of which are incorporated herein by reference.

[0054] The preceding description has been presented with reference topresently preferred embodiments of the invention. Workers skilled in theart and technology to which this invention pertains will appreciate thatalterations and changes in the described structure may be practicedwithout meaningfully departing from the principal, spirit and scope ofthis invention.

[0055] Accordingly, the foregoing description should not be read aspertaining only to the precise structures described and illustrated inthe accompanying drawings, but rather should be read consistent with andas support to the following claims which are to have their fullest andfair scope.

1. A method for measuring near-field electrical activity at a locationin a heart comprising: introducing into the heart a catheter comprising:an elongated tubular body having a distal region and a circumferentialrecess along the length of the distal region, a first electrode mountedon the distal region in close proximity to the circumferential recess,and a second electrode mounted within the circumferential recess;positioning the distal region at the location in the heart so that thefirst electrode is in direct contact with heart tissue and the secondelectrode is not in direct contact with heart tissue but is in contactwith blood; obtaining a first signal with the first electrode; obtaininga second signal with the second electrode; comparing the first signaland the second signal to obtain the near-field electrical activity atthe location in the heart.
 2. A method as claimed in claim 1, whereinthe first electrode is a tip electrode.
 3. A method as claimed in claim1, wherein the first electrode is a ring electrode.
 4. A method asclaimed in claim 1, wherein the second electrode is mounted a distancefrom the first electrode ranging from about 0.1 mm to about 2.0 mm.
 5. Amethod as claimed in claim 1, wherein the second electrode is mounted adistance from the first electrode ranging from about 0.5 mm to about 1.0mm.
 6. A method as claimed in claim 1, wherein the outer diameter of thesecond electrode is from about 20% to about 50% of the outer diameter ofthe portion of the distal region on which it is mounted.
 7. A method asclaimed in claim 1, wherein the catheter further comprises a locationsensor.
 8. A method as claimed in claim 7, further comprising acquiringnear-field electrical activity information with the first and secondelectrodes and location information with the location sensor at aplurality of locations within the heart.
 9. A method as claimed in claim8, further comprising generating a map of the electrical activity of theheart using the acquired near-field electrical activity information andlocation information.
 10. A method as claimed in claim 7, wherein thelocation sensor is an electromagnetic location sensor.
 11. A method formeasuring near-field electrical activity at a location in a heartcomprising: introducing into the heart a catheter comprising: anelongated body having an outer diameter and a distal region, a firstelectrode mounted on the distal region, and a second electrode mountedon the distal region in close proximity to and electrically isolatedfrom the first electrode, the second electrode having an outer diameterless than the outer diameter of the portion of the distal region onwhich it is mounted; positioning the distal region at the location inthe heart so that the first electrode is in direct contact with hearttissue and the second electrode is not in direct contact with hearttissue but is in contact with blood; obtaining a first signal with thefirst electrode; obtaining a second signal with the second electrode;comparing the first signal and the second signal to obtain thenear-field electrical activity at the location in the heart.
 12. Amethod as claimed in claim 11, wherein the first electrode is a tipelectrode.
 13. A method as claimed in claim 11, wherein the firstelectrode is a ring electrode.
 14. A method as claimed in claim 11,wherein the second electrode is mounted a distance from the firstelectrode ranging from about 0.1 mm to about 2.0 mm.
 15. A method asclaimed in claim 11, wherein the second electrode is mounted a distancefrom the first electrode ranging from about 0.5 mm to about 1.0 mm. 16.A method as claimed in claim 11, wherein the outer diameter of thesecond electrode is from about 20% to about 50% of the outer diameter ofthe portion of the distal region on which it is mounted.
 17. A method asclaimed in claim 11, wherein the catheter further comprises a locationsensor.
 18. A method as claimed in claim 17, further comprisingacquiring near-field electrical activity information with the first andsecond electrodes and location information with the location sensor at aplurality of locations within the heart.
 19. A method as claimed inclaim 18, further comprising generating a map of the electrical activityof the heart using the acquired near-field electrical activityinformation and location information.
 20. A method as claimed in claim17, wherein the location sensor is an electromagnetic location sensor.21. A method for measuring near-field electrical activity at a locationin a heart comprising: introducing into the heart a catheter comprising:an elongated body having a distal region, a first electrode mounted onthe distal region, and a second electrode mounted on the distal regionin close proximity to and electrically isolated from the firstelectrode, the second electrode being covered by a blood-permeablemembrane that prohibits direct contact between the second electrode andsurrounding heart tissue; positioning the distal region at the locationin the heart so that the first electrode is in direct contact with hearttissue and the second electrode is not in direct contact with hearttissue but is in contact with blood; obtaining a first signal with thefirst electrode; obtaining a second signal with the second electrode;comparing the first signal and the second signal to obtain thenear-field electrical activity at the location in the heart.
 22. Amethod as claimed in claim 21, wherein the blood permeable membranecomprises a biocompatible composition selected from the group consistingof polyolefins, polyurethane, polyetheramide, polyetherimide, polyimide,fluoropolymers, silicones and combinations thereof.
 23. A method asclaimed in claim 21, wherein the first electrode is a ring electrode.24. A method as claimed in claim 21, wherein the first electrode is atip electrode.
 25. A method as claimed in claim 21, wherein the catheterfurther comprises a location sensor.
 26. A method as claimed in claim25, further comprising acquiring near-field electrical activityinformation with the first and second electrodes and locationinformation with the location sensor at a plurality of locations withinthe heart.
 27. A method as claimed in claim 26, further comprisinggenerating a map of the electrical activity of the heart using theacquired near-field electrical activity information and locationinformation.
 28. A method as claimed in claim 25, wherein the locationsensor is an electromagnetic location sensor.
 29. A catheter comprising:an elongated body having a distal region; a first electrode mounted onthe distal region; and a second electrode mounted on the distal regionin close proximity to and electrically isolated from the firstelectrode, the second electrode being covered by a blood-permeablemembrane that, in use, prohibits direct contact between the secondelectrode and surrounding heart tissue.
 30. A catheter as claimed inclaim 29, wherein the blood permeable membrane comprises a biocompatiblecomposition selected from the group consisting of polyolefins,polyurethane, polyetheramide, polyetherimide, polyimide, fluoropolymers,silicones and combinations thereof.
 31. A catheter as claimed in claim29, wherein the first electrode is a ring electrode.
 32. A catheter asclaimed in claim 29, wherein the first electrode is a tip electrode. 33.A catheter as claimed in claim 29, wherein the catheter furthercomprises a location sensor.
 34. A catheter as claimed in claim 33,wherein the location sensor is an electromagnetic location sensor.